1. Field of the Invention
The invention concerns the optical reading of indexing marks and, more precisely, of bar codes placed on labels or, more generally, on documents or on any object with a substantially flat surface.
2. Description of the Prior Art
Indexing marks are used for the recognition of objects in order to make it easier to manage them. They are commonly used, for example, to indicate references or prices of goods. Rather than recording solely references that are clearly readable by an observer, it is attempted also (or instead) to place codes that can be easily recognized by optical detectors. The automation of certain operations may be greatly facilitated thereby. For example, a cashier in a shop will work faster by placing an optical pencil on an indexing mark on an article, than by reading the price and reference and typing them on the keyboard of a cash register.
Bar codes are marks which lend themselves particularly well to automated optical reading. They consist of rectangular strips or bars, the width and/or spacing of which is used to encode the useful information. For example, when this information, as is very frequently the case, is a sequence of decimal figures, the corresponding mark will be formed by a sequence of parallel strips with a width s.1 where 1 is the width of the narrowest strips and s is a whole number of a few units at the most, the spacing of the strips being m.e where e is the narrowest nominal space between strips and m is a whole number of a few units.
The code may also be limited to a variable spacing of strips of constant width or, on the contrary, to a variable width of strips spaced out at a constant distance.
In all cases, the reading of the bar code requires the recognition of the presence of bars, even in the presence of noise, for example, in the presence of black spots mixed with the black bars printed on a white background.
In the present invention, we shall not be concerned with decoding as such (i.e. the conversion of widths and/or spacings into a sequence of decimal figures for example) but only with the recognition of the presence of bars.
The precision of recognition depends on the marking quality and on the background against which it is placed. The bars should be printed very sharply with a sharp luminous contrast with respect to the background. The precision is also a function of the quality of optical detector used.
Special care should be attached to he marking quality if the detector is of a basic type. Certain detectors are capable only of differentiating between highly absorptive zones (very black ink for example) and a highly reflective background (white paper background for example) and again, of doing so on condition that the zones are very sharply demarcated.
In certain cases, there is so little certainty of the quality of the background on which the marks are made, because various recordings or spots are superimposed on the indexing mark that special inks are used, reflecting only one wavelength, along with optical detectors that work selectively at this wavelength: this complicates the manufacturing process and increases the cost of systems.
An object of the invention is a method for reading bar codes which can be used to recognize and decode marks even when they are imperfect or when they are placed on surfaces of poor quality. This enables simplifying the marking methods by permitting the use of ordinary printers insteads of expensive printing processes.
On the whole, standard methods for reading bar codes consist simply in making an optical detector move past the code (or conversely) at a constant speed, crosswise to the direction in which the strips are extended, in observing the existence of successive spikes of the output signal of the detector, said spikes corresponding to maximum values of absorption, transmission or reflection of light, and in comparing these peaks with predefined thresholds, to deduce therefrom the presence or absence of strips (and then their width or their spacing).
The invention proposes a far more sophisticated method used to take into account damage in marking such as breaks in bars, poor contrast, unwanted spots, uneven printing, etc.